1. Field of the Invention
The present invention relates to a technology for manufacturing a field-effect transistor on the surface layer portion of a polycrystalline film (polycrystalline semiconductor thin film), a polycrystalline semiconductor thin film substrate for manufacturing the Field-Effect Transistor, and a semiconductor device for manufacturing electronic devices, such as liquid crystal display apparatuses and information processing devices, having the Field-Effect Transistor installed therein. The present invention also relates to an annealing method, an annealing apparatus for manufacturing the semiconductor device, and a display apparatus using the semiconductor device.
2. Description of the Related Art
As a display system of a liquid crystal display (LCD), use is made of an active matrix system in which display is performed by turning on each pixel. To turn on individual pixels in the active matrix system, a kind of field-effect transistor, an amorphous silicon thin film transistor (hereinafter, referred to as “a-SiTFT”), is mostly used.
Liquid crystal displays have been investigated and developed with the view toward attaining the technical purposes: (i) improving accuracy, (ii) increasing an aperture ratio, (iii) reducing weight, and (iv) reducing cost. To attain these purposes, Field-Effect Transistor, namely, a polycrystalline silicon thin film transistor (hereinafter referred to as “poly-SiTFT”) has been recently received attention in place of the a-SiTFT. Poly-SiTFT has a field-effect mobility of carrier higher than a-SiTFT by two orders of magnitude. By virtue of this, the device formed of the poly-SiTFT can be reduced in size and a circuit can be integrated. As a result, a driving circuit and an arithmetic circuit can be mounted on a liquid crystal display.
Such a Poly-SiTFT is manufactured by an excimer laser crystallization method, which is detailed in “flat panel display” 1999, Nikkei Micro Device, Supplemental Edition (Nikkei BP Co., Ltd. 1998, pp. 132-139).
Referring now to FIGS. 1A to 1D, a method of manufacturing a poly-SiTFT by conventional excimer laser crystallization will be explained. As shown in FIG. 1A, first, an underlying layer protection film 102 (e.g. SiO2 film, SiN film, and SiN/SiO2 laminate film) and an amorphous silicon thin film 103 are sequentially deposited on a glass substrate 5. Then, as shown in FIG. 1B, when the amorphous silicon thin film 103 is irradiated with an excimer laser 50 (e.g., XeCl, KrF) having a square or rectangular shape formed by use of an optical system, the amorphous structure of the amorphous silicon thin film 103 is changed into a polycrystalline structure within an extremely short period of 50 to 100 nanoseconds through a melting/solidification step. When the excimer laser 50 is scanned over the substrate in the direction of the arrow 105, and locally and rapidly heated and cooled, a polycrystalline silicon thin film 106 is formed as shown in FIG. 1C.
Using the polycrystalline silicon thin film 106 shown in FIG. 1C, the thin film transistor shown in FIG. 1D is manufactured. On the polycrystalline silicon thin film 106, a SiO2 gate insulating film 107 is formed. Furthermore, an impurity element is doped into predetermined regions of the polycrystalline silicon thin film 106, a source and drain regions 109, 108 are formed. A channel region 106 is located between the source region 109 and the drain region 108. A gate electrode 110 is formed on the gate insulting film 107; a protection film 111 is formed; and then a source electrode 112 and a drain electrode 113 are formed. When voltage is applied to the gate electrode 110 of the poly-SiTFT, the current flowing between the source region 109 and the drain region 108 is controlled.
In general, the TFT used in a pixel under the active matrix control is required for maintaining charge but not required to have an extremely high mobility (field-effect mobility). Rather, a low off-state current (quiescent current) must be supplied. To reduce the off-state current, it is necessary to increase the channel length of the TFT so as not to reduce the aperture ratio of a pixel by reducing the electric field strength of the end of the drain region, with the result that the pixel TFT becomes relatively large.
On the other hand, it is necessary that the TFT used in a driving circuit and an arithmetic circuit is operated at a high speed. Consequently, a high mobility but the off-state current is a matter of concern. In particular, the high-speed operation can be attained effectively by reducing the channel length. Therefore, the channel length of a TFT is reduced. As a result, the size of a TFT for use in driving circuit and arithmetic circuits becomes small.
As described above, the characteristics and sizes of TFTs required for a pixel and for driving/arithmetic circuits completely differ. All these TFTs are desirably formed together on the same substrate by substantially the same step. If not, economical merits brought by the integration of different type TFTs in a liquid crystal display cannot be obtained.
However, the conventional laser annealing method mentioned above has a problem in that only a poly-Si thin film having a uniform crystallinity is formed. If different-sized TFTs are formed on the same substrate formed by the conventional method, the following problems (i) and (ii) may inevitably occur.
(i) A large TFT has a large number of grain boundaries in the channel region. As a result, the variance of voltages becomes low (off-state current is low); however, the operation is performed at a low speed.
(ii) A small TFT has a small number of grain boundaries in the channel region. As a result, the operation can be performed at a high speed; however, variance of voltage becomes large (off-state current is high).
As described above, in the conventional method, it has been impossible to form various TFTs different in size and characteristics on the same substrate.